Neptune in his current form is old and he's going against these youngsters. ..
Minus the good doctor, of course...
Still, Neptune trades a few heavy blows with Ace before he complains about his back. For someone so godly he was sidelined for the bulk of the Fishman Island Arc because he couldn't get things going. I don't see anything changing here.
I can't make myself vote for Doc Q because I don't know what he's fully capable of. Only thing people can say is: "hes on BB's crew! He haz to be strong!" but I think hes almost a non-factor until the manga proves different.
Caribou is crafty. He'd probably sneak around to take advantage when he feels one of them seems to be on their last leg.
..but he's not too bright. Ace either beats him full out or tricks Caribou into losing somehow.
Not as hard as round 2.